Will there be any game play that requires the use of a Vanduul ship in-game?

We don't see anything in the persistent universe as being 'required'; rather, there will certainly be experiences that are enabled by having a Scythe (traveling behind enemy lines) and others that different players may find improved with one (the lore includes at least one Scythe racer!) None of these things will ever be required for every player... Star Citizen is about making your own story with your own tools.